Many brace devices have been advanced to provide control for the movement of the human knee after injury, during recuperation after injury, and to optimize the protection and healing thereof. Prior devices have exhibited a conspicuous absence of satisfactory understanding of knee anatomy and especially knee kinematics. In order to appreciate the full worth of this invention, a general understanding of knee anatomy and kinematics is desirable.
The knee joint includes three bones, the patella, the femur and the tibia. The distal end of the femur consists of a bicondylar structure which refers to the two blunt projections, known as "condyles", forming the lower end of the femur. These two condyles, medial (inner) and lateral (outer), are asymmetrically cam-shaped. The proximal end of the tibia is comprised of a specialized surface termed the tibial plateaus upon which the corresponding condyles articulate.
Unlike the hip joint, where the contour of the joint is a primary stabilizing factor, a primary stabilizing factor in the knee is the surrounding supporting tissue such as the fibrous capsule with its specialized components, the capsular ligaments and the menisci. Most important to stabilization in the knee, are the two intra-articularly located cruciate ligaments.
Knee stability can be considered in terms of static and dynamic stability. The above-noted structures are important in both instances, but during motion, certain muscle units become increasingly important, not only in terms of knee joint stability, but also in terms of carrying out the normal knee kinematics.
The mechanical axis of the lower limb can be said to extend from the center of the femoral head to the center of the ankle joint, passing near to the center of the knee. The true vertical axis is a line that extends from the center of gravity of the body down in the direction of gravity in a plane perpendicular to gravity. In the normally aligned lower limb, the mean mechanical axis of the leg is angled 3 degrees toward the true vertical axis. The femoral shaft is angled downward approximately 9 degrees toward the vertical axis and the tibia is angled approximately 6 degrees outward (valgus) with relationship to the femur.
The knee is a complex joint with multiple movements. Anatomically, the knee is classified as a diarthrodial joint of the ginglymus (hinge) type; however, even this broad definition does not do justice to the complex series of movements that occur during normal knee motion.
The ginglymus connotation refers to the flexion and extension movement, but flexion and extension do not occur about a fixed transverse axis but rather about a constantly changing center of rotation. This particular aspect of knee motion has been appropriately labeled "polycentric rotation". This phenomenon; however, considered by itself, falls short of describing the kinematics of the human knee.
During flexion and extension in the sagittal plane, simultaneously abduction and adduction are occurring in the coronal plane and internal rotation and external rotation are occurring in the transverse plane. To carry the complexity further is the phenomenon of combined rolling and gliding motio of the femoral condyles on the tibial plateaus. Rolling motion determines the "roll-back" of the femur on the tibia during flexion. The ratio of rolling to gliding motion differs in the lateral compartment compared to the medial. This kinematic fac gives rise to the phenomenon known as "differential roll-back."
The knee joint is often times subjected to a loading force equal to several times the body weight in level walking. These forces increase with running or other "impact loading" activities. Loads are not transmitted over the joint surface equally but rather over a relatively small area of each femora condyle and the tibial plateau. The medial side of the joint bears a larger load than the lateral; but the medial plateau is also larger than the lateral; therefore, the force per unit ar is approximately equal.
The configuration of the femoral condyles is asymmetric. The lateral condyle is broader in the sagittal an transverse planes than the medial condyle The medial condyle projects distally to a level slightly lower than the lateral. This distal projection helps to compensate for the varus (toward) inclination in the femur with respect to the vertical axis. As a result, in the erect, in-line position, the transverse plane of the condyles lies near the horizontal.
As the knee approaches full extension, it can be considered that the femur rotates internally (as concurrently the tibia is rotating externally) allowing the anterior articular surface of the medial femoral condyle to come in contact with the anterior portion of the medial tibial plateau. The lateral condyle moves anteriorly (forward) more rapidly than the medial, thus producing the phenomenon of the "screw home mechanism", until the knee is "locked" in the fully extended position. This rotary movement passes through a series of polycentric axes.
The workers in the prior art have utilized a variety of hinge members in an attempt to track the sliding and roll-back of the condyles with respect to the tibia plateaus. However, none of the prior art has accomplished an external knee hinge mechanism that accounts simultaneously for the differential roll-back, the rotation of the tibia with respect to the femur during flexion and extension, and the abduction/adduction movement that occurs concurrently with the other movements. A preferred embodiment of this invention has accomplished this through a series of slotted curvilinear shells all of which have their concave surfaces facing in the same direction.